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In nine out of the 10 burger samples, the horse DNA was found at very low levels, the authority said, but in one sample from Tesco, Britain's largest retailer, the horsemeat accounted for about 29% of the burger.

Tesco responded by pulling from its shelves all products from the company that had supplied the dubious burgers.

"We understand that many of our customers will be concerned by this news, and we apologize sincerely for any distress," Tesco said.

The retailer is working with Irish and British authorities and the supplier to work out what had happened, it said.

Alan Reilly, the chief executive of the Irish food authority, said there was a "plausible explanation" for the pig meat finding its way into the burgers, since meat from different animals is processed at the same plants. But he said there was "no clear explanation at this time" for the presence of the horsemeat.

"In Ireland, it is not in our culture to eat horsemeat and therefore, we do not expect to find it in a burger," Reilly said. "Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."

Many British and Irish people expressed their distaste over the revelations on social media.